Three dimensional optical imaging systems, hereinafter referred to as “3D cameras”, that are capable of providing distance measurements to objects and points on objects that they image, are used for many different applications. Among these applications are profile inspection of manufactured goods, CAD verification, robot vision, geographic surveying and imaging objects selectively as a function of distance.
Some 3D cameras provide simultaneous measurements to substantially all points of objects in a scene they image. Generally, these 3D cameras comprise a light source, such as a laser, which is pulsed or shuttered so that it provides pulses of light for illuminating a scene being imaged and a gated imaging system for imaging light from the light pulses that is reflected from objects in the scene. The gated imaging system comprises a camera having a photosensitive surface, hereinafter referred to as a “photosurface”, such as a CCD camera, and a gating means for gating the camera open and closed, such as an optical shutter or a gated image intensifier. The reflected light is registered on pixels of the photosurface of the camera only if it reaches the camera when the camera is gated open.
To image a scene and determine distances from the camera to objects in the scene, the scene is generally illuminated with a train of light pulses radiated from the light source. For each radiated light pulse in the train, following an accurately determined delay from the time that the light pulse is radiated, the camera is gated open for a period of time hereinafter referred to as a “gate”. Light from the light pulse that is reflected from an object in the scene is imaged on the photosurface of the camera if it reaches the camera during the gate. Since the time elapsed between radiating a light pulse and the gate that follows it is known, the time it took imaged light to travel from the light source to the reflecting object in the scene and back to the camera is known. The time elapsed is used to determine the distance to the object.
In some of these 3D cameras, only the timing between light pulses and gates is used to determine the distance from the 3D camera to a point in the scene imaged on a pixel of the photosurface of the 3D camera. In others, the amount of light registered by the pixel during the time that the camera is gated open is also used to determine the distance. The accuracy of measurements made with these 3D cameras is a function of the rise and fall times and jitter of the light pulses and their flatness, how fast the gating means can gate the camera open and closed.
A 3D camera using a pulsed source of illumination and a gated imaging system is described in “Design and Development of a Multi-detecting two Dimensional Ranging Sensor”, Measurement Science and Technology 6 (September 1995), pages 1301-1308, by S. Christie, et al, and in “Range-gated Imaging for Near Field Target Identification”, Yates et al, SPIE Vol. 2869, p 374-385 which are herein incorporated by reference.
Another 3D camera is described in U.S. Pat. No. 5,081,530 to Medina, which is incorporated herein by reference. A 3D camera described in this patent registers energy in a pulse of light reflected from a target that reaches the camera's imaging system during each gate of a pair of gates. Distance to a target is determined from the ratio of the difference between the amounts of energy registered during each of the two gates to the sum of the amounts of energy registered during each of the two gates.
A variation of a gated 3D camera is described in U.S. Pat. No. 4,935,616 to Scott, which is incorporated herein by reference. In this patent, a 3D camera is described in which a light source and imaging system, instead of being fully gated, are “modulated”. In a preferred embodiment of the invention, the light source comprises a CW laser. The intensity of light radiated by the laser is modulated so that the intensity has an harmonically varying component. The sensitivity of the camera's imaging system to light is also harmonically modulated. When a target that is illuminated by the modulated laser light reflects some of the incident laser light, the reflected light has the same modulation as the laser light. However, modulation of the reflected light that reaches the imaging system from the target has a phase difference with respect to the modulation of the imaging system that depends upon the distance of the target from the camera. The intensity that the camera registers for the reflected light is a function of this phase difference. The registered intensity is used to determine the phase difference and thereby the distance of the target from the camera.
Other “gated” 3D cameras and examples of their uses are found in PCT Publications WO97/01111, WO97/01112, and WO97/01113 which are incorporated herein by reference.
An optical shutter suitable for use in 3D cameras is described in PCT patent application PCT/IL98/00060, by some of the same applicants as the applicants of the present application, the disclosure of which is incorporated herein by reference.